1. Field of Invention
This invention is directed to rendering patterns in place of colors in order to represent the colors in a black and white system.
2. Description of Related Art
Color images are increasingly used to convey information. However, support for color representation is not always provided in black and white facsimile machines, photocopy machines and other image output devices. Some conventional systems provide for black and white representation of color in which a pattern is selected to represent a color in the original color palette. A pattern representing the color is then reproduced. Since differing colors require different patterns, conventional techniques for providing this mapping of color to pattern require the calculation of many unique patterns. This requires time and/or circuitry for the necessary calculations.
Alternatively, other conventional systems have used memory structures to store the patterns as the patterns are generated. This technique reduces the time to calculate the patterns after the first color is encountered, since the calculations used to create the patterns are performed once for each color and stored in a buffer. However, such systems have difficulty in allocating sufficient memory when processing complex documents in which many colors and shades are used. For complex color documents, the storing and retrieval functions also consume processing cycles and/or increase circuitry complexity. This increases the time and/or cost necessary to create a document. Also, the amount of memory available to store all the pattern entries may be exceeded by complex color documents.
These problems limit the number of colors that can be handled and may require additional processing cycles to handle out-of-memory conditions, further increasing the time and cost necessary to create an output page. Furthermore, due to the interaction of the patterns, grayscale control is difficult and non-monotonic grayscales are occasionally generated. Non-monotonic grayscales create perceptual problems for users since the intuitive correspondence between the color and the pattern is not maintained.
This invention provides systems and methods that preserve the lightness relationship between colors in a document.
This invention provides systems and methods that reduce processing cycles and/or increase the processing speed for processing a black-white version of a color document.
This invention separately provides systems and methods that reduce the memory requirements for implementing black-white printable versions of a color document.
In various exemplary embodiments of the systems and methods according to this invention, the required memory is reduced by using the hue angle of each color to be printed as an index into a table of halftones. The indexed halftone information is then combined with the luminance, or lightness, information for the indexed color to render a color in a black/white printable version with increased processing speed and/or without having to store color pattern information.
In various exemplary embodiments, if a gray color is to be rendered, the gray scale halftone is selected to render the color appropriately without analyzing the hue angle.